It is well known that the interior of a can body must be supplied with a charge of compressed air during stripping from the body maker punch to avoid collapse of the can body. The need for a compressed air charge stems from the formation of a vacuum as the male punch tool is rapidly withdrawn from the can body. This vacuum would collapse the can body were it not for the timely delivery of a small charge of compressed air. Furthermore, it is believed that the compressed air assists with the removal of the can body by pushing the body off the end of the tool and also slightly expanding the body by virtue of hoop stresses set up by the hydrostatic pressure, thus relieving some friction between can and tool.
In one known apparatus, the air supply from the factory main is, following conditioning, passed to a pilot assisted electronic shuttle via an air receiver. The shuttle valve is controlled by an Autotech controller. Because of the delays in reaction time for the valve and the propagation time along the pipe lines, the point in the cycle at which the signal needs to be sent is dependent on the machine speed. At 500 cans per minute the advance in the signal timing can amount to about 165 degrees of crank movement. The opening response time of the valves is not constant and a safety margin has to be incorporated into the advance timing.
In another apparatus, known from GB-A-2268898 a mechanical valve located in the punch sleeve is opened by the doming tool for the base profile of the can body and re-closed as the punch is withdrawn by virtue of its inertia.